While it is well known that waste oil has a high caloric content, it has not been an effective heating medium because it tends to clog conventional nozzles. It proved undesireable because it produces dense black smoke which is objectionable for environmental purposes and has a fixed reputation for clogging the interior of burner boxes, fouling the works and preventing effective operation thereafter. Although numerous attempts have been made to utilize a combination of conventional oil and waste oil, such efforts have thus far been unsuccessful except in very limited applications.